Trading Energy for Speed and Skill

One way to introduce more flexibility into standard spellcasting is to allow casters to exchange extra energy for special benefits. Below are two examples; see Adjustable Spells (this page) for an additional option. All such rules give magic-workers who have lots of spare energy (high personal FP, piles of Powerstones, etc.) a considerable advantage. [...]

Faster Casting: Wizards can reduce casting time by one second per 4 extra energy points spent. This option can’t reduce casting time below a second, and isn’t available for ceremonial magic.
Increased Effective Skill: The energy-for-skill tradeoff of Ceremonial Magic (p. B238) applies to all castings: +1 to skill for 20% extra energy, +2 for 40%, +3 for 60%, +4 for 100%, and another +1 per additional 100% of the required energy. Such skill
increases affect the roll to cast the spell and the Power of enchanted items, but they don’t reduce casting time, energy cost, or ritual requirements. A further option for Missile spells is to give +1 to the attack roll per extra energy point spent.

[Source GURPS 4th Edition Thaumatology p.39]

Does this mean that if a Spell costs 1 to cast and I have skill 20 which reduces casting cost by 2 that I can "virtually" spend +100% on the spell (i.e. 2FP - 2FP = 0FP) to get the +4 bonus to the effective roll?

The question could also be formulated as: Is this extra energy part of the casting cost?

1 Answer
1

But it worth noting that this is an option, which is specifically an example for GMs wanting to give casters a way to use excess energy. All of the options and suggestions in Thaumatology should be considered carefully by the GM before including them in his or her game, and this is mentioned several times throughout the book.

From the introduction:

As with any toolbox, you shouldn’t try to use everything
simultaneously – and by “you,” we mean whoever is
responsible for designing and maintaining the campaign,
usually the GM. Pick and choose only what suits your purposes.
Not every concept belongs in every game; if you try
to use it all, you’ll probably go crazy and, worse, your campaign
will collapse!
All of Thaumatology is one big suggestion.
For example, the point costs for abilities reflect the
author’s views of effectiveness and balance – but as the GM,
you’re free to disagree and change the numbers(...)

But if the GM decides that a section or even a chapter doesn’t apply, then that’s final.

-- GURPS Thaumatology, p. 4

And at the start of the Minor Spell Variants chapter, it is stated (emphasis mine):

What follows are some suggestions for minor changes to standard, spell-based magic(...)

-- GURPS Thaumatology, p. 19

The implication is that not everything in Thaumatology necessarily balanced, and might even be broken under certain circumstances. The side-effect of giving players with high skill free skill bonuses doesn't really seem in the spirit the option, although probably not game-breaking.

Personally as a GM, I would rule a minimum 1 point of energy per +1 bonus if I were to use this option.